Putting any 2FA behind a paywall makes it more inaccessible for users, especially if the version put behind the paywall is as widely used as text-based authentication. Fewer people may be inclined to set it up, or they may be ignoring the pop-ups from Twitter to update their accounts so that they can get back to tweeting. This may result in more compromised accounts, and consequently make Twitter a less secure platform with more potential for attacks and impersonation.
